The invention of the subject application pertains to an array induction logging apparatus for oil well boreholes and more particularly, to a borehole correction system associated with the induction logging apparatus for correcting for the effects of the borehole on an overall formation conductivity measurement.
An induction logging apparatus, disposed in a wellbore, or borehole of an oil well, basically comprises at least one transmitting coil and at least one receiving coil mounted on a support and axially spaced from each other in the direction of the borehole. The transmitting coil is energized by an alternating current at a frequency which is typically 20 kHz and generates an electric field which induces in the formation, surrounding the borehole, eddy currents which flow coaxially to the borehole and the intensity of which is proportional to the conductivity of the formation. The field generated in turn by these eddy currents induces in the receiving coil an electromotive force (EMF), which produces a received signal in the receiving coil. By suitably processing the received signal from the receiving coil, a measurement of the conductivity of the formation is obtained.
However the conductivity of the mud or drilling fluid in the borehole may distort the measured value of the conductivity of the formation. One prior art induction logging apparatus, known as the 6FF40 sonde, consists of three transmitter coils and three receiver coils. In the 6FF40 sonde, the effective coil spacing is about 40 inches; the coil positions and the number of turns on the coils is designed to minimize the effect of the borehole. Therefore, although the conductivity of the mud (sigma.sub.m or s.sub.m) did distort the determination of the true formation conductivity (Sigma.sub.t or s.sub.f) the distortion was small and required a relatively minor correction. As a result, with the aforementioned prior art well logging apparatus, the borehole corrections could be applied after recording and signal processing the data, and frequently were left up to the formation analyst interpreting the induction logs. However, a new well logging array induction tool was developed, the characteristics of which were disclosed in prior pending application Ser. No. 043,130, filed Apr. 27, 1987 entitled "Induction Logging Method and Apparatus", and in prior pending application Ser. No. 932,231, filed Nov. 18, 1986 entitled "Induction Logging Sonde With Metallic Support", now U.S. Pat. No. 4,873,488. These applications, Ser. Nos. 043,130 and 932,231, are incorporated by reference into the specification of this application. This new well logging array induction tool (hereinafter termed the "AIT Tool") has one transmitter coil and nine (9) receivers with two coils per receiver. The spacing between transmitter and receiver coil pairs ranges from 6 inches to 72 inches. The signals from the nine receivers collect information about the conductivity at different depths in the formation. Because there are so many more measurements in the new AIT tool, relative to the prior art induction logging apparatus, manual correction of the signal for the effect of the borehole and would be cumbersome and time-consuming. Also in the AIT the receivers with a small spacing are more strongly affected by the conductive borehole fluid, and the correction is not a small fraction of the recorded signal. As a result, using the new AIT tool, in order to obtain an accurate measurement of the true conductivity of the formation s.sub.t, the measured EMF for each receiver must be corrected for the effect of the borehole in software, prior to signal processing. The portion of the induced EMF associated with the conductivity of the mud, s.sub.m, must be removed from the induced EMF in each of the nine receivers thereby yielding an EMF which represents the actual conductivity of the formation. The induced EMF signals form the AIT, after applying borehole corrections, and after signal processing, give an indication of the conductivity in the formation at different radial distances from the borehole. When the AIT induction logs are combined with other information such as the porosity of the rock, and the ground water conductivity, they can be used to infer the water saturation which indicates the presence of hydrocarbons. Because the AIT probes different radial distances into the formation, it will also indicate invasion of the formation rock by fluids from the borehole.
A prior art technique for correcting an induction log for the effect of the borehole is set forth in a publication entitled "Phasor Induction Tool", published by the assignee of this application, dated July 1986. A similar prior art technique is set forth in another publication entitled "Schlumberger Log Interpretation Chart", the 1979 edition. In both the aforementioned publications, the described prior art technique for performing the borehole correction involves manually performing the subtraction by referring to set of curves (borehole geometrical factor versus hole diameter), calculating the contribution to the induced EMF associated with the conductivity of the mud, and performing the aforementioned subtraction.
This prior art manual technique would be very cumbersome for an array induction sonde having many receiving coils, such as the new AIT tool. Furthermore it would not be sufficiently accurate for the signals coming from the short-spacing receivers.